FIG. 8 hereof schematically illustrates a general setup of a conventional electric power steering apparatus 100 suitable for application to a motor vehicle. The conventional electric power steering apparatus 100 includes a steering wheel 101, a steering shaft 102 connected integrally to the steering wheel 101, a manual steering torque generation mechanism 106 provided on the steering shaft 102, and a rack and pinion mechanism 105 having a pinion 105a coupled to the manual steering torque generation mechanism 106 via a connecting shaft 103 having universal joints 103a and 103b. The rack and pinion mechanism 105 includes a rack shaft 107 having a rack tooth 107a meshing with the pinion 105, and the rack shaft 107 can axially reciprocate via the meshing engagement between the rack teeth 107a and the pinion 105. Left and right front wheels 109, constructed as steerable wheels of the vehicle, are coupled via tie rods 108 to the opposite ends of the rack shaft 107. Human operator or driver of the vehicle operates the steering wheel 101 to steer the steerable front wheels 109 by way of the manual steering torque generation mechanism 106 and rack shaft 107.
To reduce manual steering torque to be generated via the manual steering torque generation mechanism 106, an electric motor 110 for supplying steering assist torque is provided, for example, coaxially with the rack shaft 107. Rotational force supplied as the Steering assist torque by the motor 110 is converted into linear force via a ball thread mechanism 111 provided substantially parallel to the rack shaft 107, which is applied to the rack shaft 107. Rotor of the steering assisting motor 110 has a helical driving gear 110a provided integrally thereon and meshing with a helical gear 111b provided integrally on an end of a threaded shaft 111a of the ball thread mechanism 111. The ball thread mechanism 111 has a nut operatively coupled to the rack shaft 107.
FIG. 9 hereof is a block diagram showing a control device employed in the conventional electric power steering apparatus 100. Within a steering gearbox (not shown in FIG. 8), there is provided a manual steering torque detector section 112 for detecting manual steering torque T acting on the pinion 105a. The manual steering torque detector section 112 converts the detected manual steering torque T into a manual steering torque detection signal Td and delivers the thus-converted manual steering torque detection signal Td to the control device 114. Using the manual steering torque detection signal Td as a primary motor-operating signal, the control device 114 operates the steering assisting motor 110 and controls steering assist torque to be produced by the motor 110. The control device 114 includes a target current setting section 115 and a controller 116. The target current setting section 115 sets target assisting torque on the basis of the manual steering torque detection signal Td and generates a target motor current signal IT necessary for the motor 110 to produce the target assisting torque.
FIG. 10 is a block diagram showing detailed structure of the controller 116 of FIG. 9. As shown, the control section 116 includes an offset calculation section 117, a motor operation control section 118, a motor drive section 119, and a peak current value detection section 120. The offset calculation section 117 calculates an offset value between the target motor current signal IT output from the target current setting section 115 of FIG. 9 and a peak current signal IM output from the peak current value detection section 120, and it outputs an offset signal 117a indicative of the calculated offset value. The motor operation control section 118 includes an offset current control section 121 and a PWM (Pulse Width Modulated) signal generation section 122. The offset current control section 121 performs processing, such as proportional, integral and differential (PID) operations, on the offset signal 117a supplied from the offset calculation section 117 and thereby generates a driving current signal 121a for controlling the motor current to be supplied to the motor 110 in such a manner that the offset signal 117a approaches an ideal zero value. The PWM signal generation section 122 generates a PWM signal for PWM-operating the motor 110 on the basis of the driving current signal 121a and outputs the thus-generated PWM signal as a drive control signal 122a. 
Further, in the controller 116, the motor drive section 119 includes a gate-driving circuit section 123, and a motor drive circuit section 124 having four powering FETs (Filed Effect Transistors) interconnected via an H-shaped bridge circuit. The gate-driving circuit section 123 selects two of the four FETs on the basis of the drive control signal (PWM signal) 122a and in accordance with a current steering direction of the steering wheel 101, and it drives the gates of the selected two FETs to allow these FETs to perform a switching operation. The peak current value detection section 120 detects a peak value of the motor current (armature current) flowing through the steering assisting motor 110 and outputs a peak current signal IM. In the manner set forth above, the control device 114 PWM-controls the current to be supplied from a battery power supply 126 to the motor 110 and thereby controls the output power (steering assist torque) of the motor 110, on the basis of the manual steering torque T detected via the manual steering torque detector section 112 of FIG. 9.
As seen in FIG. 10, the control device 114 achieves enhanced control characteristics of the motor 110 by the controller 116 detecting the peak value of the motor current actually flowing through the motor 110 and performing feedback control of the motor current based on the peak current signal IM. In the aforementioned manner, the manual steering torque T applied by the vehicle driver is detected via the manual steering torque detector section 112 of the manual steering torque generation mechanism 106 shown in FIG. 8, and the control device 114 controls the output power of the motor 110, on the basis of the detected manual steering torque T, so as to assist the rack shaft 107 in the steering gearbox in moving linearly for desired steerage.
Control method performed by the conventional control device 114 is explained below. As illustrated in FIG. 10, the steering assisting motor 110 is driven on the basis of feedback control of the motor current performed by the control device 114. Then, the motor current flowing through the motor 110 when two diagonally-opposed FETs, among the four FETs interconnected by the H-shaped bridge circuit, are in the ON state is detected by the peak current value detection section 120.
FIG. 11 is a block diagram showing the peak current value detection section 120, motor drive circuit 124, steering assisting motor 110 and battery 126. The peak current value detection section 120 generates the peak current signal IM on the basis of a voltage VS(t) across both ends of a shunt resistor 125 connected in series with the motor drive circuit 124. The peak current value detection section 120 includes a peak holding circuit for holding a peak value VP of the voltage VS(t) input thereto. The peak current signal IM output from the peak current value detection section 120 represents a peak value of the motor current (detected peak current value) ISP. Switch 127 is kept in a closed state during operation of the vehicle, so that voltage of the battery 126 is applied to a capacitor 128. The capacitor 128 is provided to stabilize the battery voltage to be fed to the motor drive circuit 124. The peak current value ISP (peak current signal IM) detected by the peak current value detection section 120 is fed back to the offset calculation section 117 for calculation of an offset between the target motor current signal IT and the peak current value ISP, and the driving of the motor 110 is controlled so that the offset is minimized to zero.
With the electric power steering apparatus where the feedback control is performed using the peak-holding-type peak current value detection section 120, there would arise the following inconveniences.
In the peak current value detection section 120 using the peak holding circuit to perform the current detection, the peak value is detected from the motor current varying in response to a duty cycle of the PWM signal. Thus, when the duty cycle of the PWM signal is smaller than 50% so that the motor current takes a small value, a difference between an average value of the actual motor current and the detected peak current value would become considerably great. The following paragraphs set fourth relationship between the duty cycle of the PWM signal and the detected peak current value and inconveniences arising from the relationship.
FIGS. 12 and 13 are timing charts each illustrating variations over time of various current values (i.e., value of the motor current, average value of the motor current, shunt current value and detected peak current value) relative to the PWM signal. Specifically, FIG. 12 shows a timing chart when the duty cycle of the PWM signal is greater than 50% and the motor current is relatively great. More specifically, section (a) of FIG. 12 shows a variation over time of the PWM signal 122a output from the PWM signal generation section 122, and section (b) of FIG. 12 shows variations over time of the motor current IMM flowing through the motor 110, average value Ia of the motor current, shunt current Ish and peak current IM detected by the peak current value detection section 120 (detected peak current value ISP). In this case, a ratio of the peak value of the motor current (i.e., detected peak current value ISP) to the average value Ia of the motor current, namely, ISP/Ia, is very close to a value “1”; that is, the average value Ia and of the motor current and the detected peak current value ISP are close to each other.
FIG. 13 shows variations over time of the various current values when the duty cycle of the PWM signal is less than 50% and the motor current is relatively small. Specifically, section (a) of FIG. 13 shows a variation over time of the PM signal 122a output from the PWM signal generation section 122, and section (b) of FIG. 13 shows variations over time of the motor current IMM flowing through the motor 110, average value Ia of the motor current IMM, shunt current Ish and peak current IM detected by the peak current value detection section 120 (detected peak current value ISP). In this case, the ratio, to the average value Ia of the motor current, of the peak value of the motor current (i.e., detected peak current value ISP), namely, ISP/Ia, is considerably greater than the value “1”; that is, the average value Ia and of the motor current and the detected peak current value ISP are greatly different from to each other. In this case, the detected peak current value ISP to be used for the feedback control, greatly differing from the average value Ia of the motor current, would prevent optimal feedback control of the motor current.
FIG. 14 is a graph showing characteristics of the detected peak current value ISP relative to the average value Ia of the motor current, where the horizontal axis represents the average value Ia while the vertical axis represents the detected peak current value ISP. As shown, when the average value Ia of the motor current is small, the ratio ISP/Ia is greater than the value “1”, presenting a nonlinear characteristic. Therefore, according to the conventional control method performed by the control device 114 based on the feedback control using the peak-holding-type peak current value detection section 120, there are obtained actual control characteristics as illustratively shown in FIG. 15. In FIG. 15, the horizontal axis represents the target motor current signal IT, while the vertical axis represents the average value Ia of the motor current. From FIG. 15, it is seen that the average motor current value Ia becomes smaller than the value of the target motor current signal IT without coinciding with the latter. Therefore, at the beginning of turning, by the vehicle driver, of the steering wheel, when the target motor signal IT is set at a small value (i.e., a small value range), the average value Ia of the motor current would come short of a predetermined value so that the motor current can not be output as designated by the target motor signal IT, as a result of which a desired steering feel can not be attained.
Namely, with the conventional control device 114 where the peak motor current, i.e. the detected peak current value ISP, is ted back by the peak current value detection section 120 for the motor current control, there would arise the problem that, when the target motor current signal is set at a small value (i.e., a small value range), the actual steering assisting current (average motor current value) would become smaller than the target steering assisting current (target average motor current value), undesirably providing insufficient torque assist.